Switched to the coveralls module for shipping coverage reports

Switching to [node-coveralls](https://github.com/cainus/node-coveralls) brought a few new features with it:
 - git data is sent to coveralls (commit hash, branch name, etc)
 - integration with more CI services (codeship, jenkins, circleci, travis)
 - pulls information from the environment that is provided by these services, making the coveralls option unnecessary in most cases.

In order to support testing in my current repo, added this option (willing to submit seperately)
 - Moved `coverage` option to `instrument`, now it will not be overridden when `coverage`/`coveralls` is set to true. `coverage` is no longer documented, but is still supported. Allows the user to instrument code themselves.

Known issue:
 - When a user sets the repoToken via the options, this occurs: cainus/node-coveralls#33

Closes #23